The present invention is related to optical networking and more particularly to systems and methods to support proper regulation of optical signal power levels.
Wavelength division multiplexing (WDM) techniques have been developed to increase the capacity of optical networks by combining multiple optical signals of disparate wavelengths onto the same fiber. Each wavelength constitutes an independent channel for communication. To allow optical signals including composite WDM optical signals to extend over large distance without costly recovery of the transmitted data and regeneration of the optical signals, optical amplifiers have been developed. Improvements in optical amplifier bandwidth permit large numbers of WDM wavelengths to be amplified simultaneously by a single amplifier.
Key considerations in optical link design include the placement of optical amplifiers along the link, setting the nominal amplifier gain, and adjusting amplifier power in accordance with current operating conditions. The objective is to regulate the received optical signal power and signal to noise ratio (OSNR) on each channel. Each OSNR should be sufficient for accurate recovery of transmitted data yet the optical signal power should not be so high so as to exceed the dynamic range of receiver components. Optical power levels at amplifier outputs should be sufficiently low to avoid undesirable non-linear effects. Proper receiver OSNR may be assured by specifying an appropriate per channel output power for each amplifier along the link. This output power per channel should be maintained even as the number of channels carried and link losses vary over time.
One way to assure constant output power per channel is to measure and regulate power on a per-channel basis at each amplifier. However, economic constraints mandate that the aggregate power be controlled to avoid the need to break the composite WDM signal into its wavelength components. To address variations in the number of channels, one could simply select and maintain an appropriate constant gain (output power over input power) without needing to be cognizant of the current channel count. However, this strategy would not maintain constant output power per channel in the face of variation in losses prior to the amplifier. Variations in losses can be addressed by maintaining total output power but then changes in channel count will lead to changes in output power per channel.
An optical amplifier power regulation technique has been developed to address the need to maintain per channel output power in the face of variations in channel count and link loss characteristics. In this technique, an amplifier power control processor is cognizant of the number of WDM channels being carried through the amplifier. A desired gain is calculated based on the maximum output power, current input power, and current channel count. This gain is recalculated whenever the channel count changes and the amplifier maintains this gain between updates.
It will be appreciated that this procedure requires that information on the current channel count be maintained and distributed through an optical link. It will also be appreciated that the channel count may change over the extent of a link as channels are dropped and/or added. Specific techniques for maintaining and distributing accurate channel counts for the purpose of amplifier power control are explained in International Publication No. WO 99/21302 published on Apr. 29, 1999, the contents of which are herein incorporated by reference in their entirety for all purposes.
A problem that arises in implementing this amplifier power control technique is that a portion of the power present at both the amplifier input and output results from a special kind of noise, ASE (Amplified Spontaneous Emission), rather than the WDM signal power. A given amplifier can be aware of its own ASE contribution and therefore take account of it when adjusting its gain. However, there is also ASE noise power resulting from previous amplifiers that is present at the amplifier input. This input ASE noise power is amplified and forms a part of the total amplifier output power. It is very difficult to locally measure input ASE power but failing to account for it results in an overestimate of output signal power, leading the amplifier to set its gain too low to achieve the desired per channel output power.
What is needed are systems and methods for maintaining and distributing information about ASE power in an optical link to support proper regulation of amplifier power.